The present invention relates to a technique of auto-focus (AF) control in an optical apparatus such as a video camera, a digital still camera, and an interchangeable lens apparatus.
An AF function provided for optical apparatuses is realized by generating a focus signal (hereinafter referred to as an AF evaluation value signal) representing the sharpness of video image from a video signal generated by an image-pickup part including an image-pickup element and then controlling drive of a focus lens such that the AF evaluation value signal is at the maximum. Such an AF method is called a contrast AF method or a TV-AF method.
The AF evaluation value signal is typically generated on the basis of high-frequency components extracted from the video signal through a band-pass filter (BPF). When an image is blurred, the high-frequency components, that is, the AF evaluation value signal is at a low level. As the image is brought into an in-focus state, the level of the AF evaluation value signal becomes higher. When an in-focus point is reached, the AF evaluation value signal is at the highest level. The characteristic of the AF evaluation value signal can be used to perform accurate control of drive of the focus lens (focus control, also referred to as AF control).
In actual AF control, when the AF evaluation value signal is at a low level, the focus lens is driven as fast as possible in a direction in which the level thereof is increased (also referred to as mountain-climbing drive), and as the level of the AF evaluation value signal becomes higher, the focus control is performed at a lower speed. In addition, to determine the direction in which the level of the AF evaluation value signal is increased, that is, the direction in which the focus lens is driven, the focus lens is minutely driven (minute-drive) and the change in the AF evaluation value signal during the minute-drive is monitored (see Japanese Patent Laid-Open No. H02 (1990)-140074). This allows the focus lens to be moved to the in-focus point in a short time.
In recent years, a higher magnification of an image-pickup lens, and an increased number and a higher density of pixels of an image-pickup element have promoted the use of cameras capable of picking up images with a higher degree of definition (resolution) as in a high-definition TV system (hereinafter referred to as a high-definition system) as well as a standard TV system such as NTSC and PAL. For picking up images in the high-definition system, the AF control can also be performed by using the abovementioned AF evaluation value signal.
In the camera capable of picking up images in the high-definition system, however, the following problems occur when the AF control is performed by using the AF evaluation value signal in the same frequency band as that in the standard TV system.
FIG. 11A shows a comparison between a resolution spatial frequency in picking up images with the standard TV system (the NTSC system is used in this case) and a resolution spatial frequency in picking up images by using a higher number and a higher density of pixels with the high-definition system. In FIG. 11A, ‘NTSC’ (Hz) represents the resolution spatial frequency in the NTSC image pickup, while ‘HD’ (Hz) represents the resolution spatial frequency in the high-definition image pickup. The ‘HD’ (Hz) is higher than the ‘NTSC’ (Hz).
Each of the resolution spatial frequencies in this case is not a resolvable limit of frequency but a spatial frequency with a sufficiently high MTF as indicated by arrows in FIG. 11A. Typically, an adequate MTF can be provided by setting approximately 80% of the resolvable limit of spatial frequency.
With the difference in the resolution spatial frequency, it is possible that the AF evaluation value signal in the ‘NTSC’ (Hz) is used for AF control in the high-definition image pickup but focus cannot be achieved. This is because the highest level of the AF evaluation value signal cannot be detected for an object image in the ‘HD’ (Hz).
FIG. 11B shows an example in which in-focus detection is performed with the AF evaluation value signals of the ‘NTSC’ (Hz) and ‘HD’ (Hz). A curve with ‘low extraction frequency’ represents the AF evaluation value signal in the ‘NTSC’ (Hz), while a curve with ‘high extraction frequency’ represents the AF evaluation value signal in the ‘HD’ (Hz).
The AF evaluation value signal in the ‘NTSC’ (Hz) has the shape of a gentle hill and the AF evaluation value signal in the ‘HD’ (Hz) has the shape of a steep hill. FIG. 11B also shows in-focus accuracy necessary for the NTSC image pickup as ΔNTSC and in-focus accuracy necessary for the high-definition image pickup as ΔHD. Since the peaks of both of the AF evaluation value signals fall within the ranges of ΔNTSC and ΔHD needed in the associated image-pickup systems, favorable in-focus accuracy can be provided in each of the image-pickup systems. However, if the AF evaluation value signal in the ‘NTSC’ (Hz) is used in the high-definition image pickup, satisfactory in-focus accuracy may not be realized since the ΔNTSC range is wider than the ΔHD range.
To address this, it is contemplated as shown in FIG. 11C that the AF evaluation value signal in the ‘NTSC’ (Hz) can be added to the AF evaluation value signal in the ‘HD’ (Hz) (that is, they are synthesized) to ensure in-focus accuracy in the high-definition image pickup.
However, constantly adding the AF evaluation value signal in the ‘NTSC’ (Hz) to the AF evaluation value signal in the ‘HD’ (Hz) during AF control may not result in favorable AF performance.
When images are actually picked up, a minor subject such as a background often exists in a frame other than a major subject. If the focus lens is moved to the front or back of the in-focus point for the major subject, the AF evaluation value signal is not simply increased or reduced as shown in FIG. 11C in many cases.
FIG. 12A shows an example in which a person as a major subject exists at the center of a frame, a mountain as a background exists behind the person, and an object exists in front of the person. FIG. 12B shows changes in an AF evaluation value signal for high definition (a high extraction frequency) and an AF evaluation value signal for NTSC (a low extraction frequency) at various positions of a focus lens. In this case, in comparison with the AF evaluation value signal for NTSC, the AF evaluation value signal for high definition has steep hills due to the effects of the background and the object in front of the person.
When these AF evaluation value signals are always synthesized in AF control, the synthesis AF evaluation value signal has two peaks on both sides of the peak corresponding to the major subject (on the closest side and infinity side) as shown in FIG. 12C. If the focus lens is driven, for example from the closest end or infinity end in AF control using the synthesis AF evaluation value signal, the focus lens is stopped at the peak for the background or the object in front of the person, not at the peak for the major subject which should be focused on. This reduces the responsiveness in AF.
When a small or a fine subject is included in the background, the AF evaluation value signal for the background is likely to be at a higher level. Particularly, the AF evaluation value signal in a high frequency band to realize in-focus accuracy for high-definition images tends to have a higher level.
For example, when many trees form a background to produce a higher frequency band as shown in FIG. 13A, the level of the AF evaluation value signal for the background is higher than the level of the AF evaluation value signal for the major subject in the high frequency band for high-definition images as shown in FIG. 13B. If these AF evaluation value signals are always synthesized in AF control, the highest mountain is formed at the focus point corresponding to the trees as the background in the synthesis AF evaluation value signal as shown in FIG. 13C. In this case, the background is more likely to be brought into focus than the major subject to cause so-called near and far subjects in the frame.
In this manner, only the AF evaluation value signal in the low frequency band for the NTSC system cannot provide in-focus accuracy suitable for the high-definition system. In addition, simply synthesizing the AF evaluation value signal for the high-definition system and the AF evaluation value signal for the NTSC system to provide in-focus accuracy necessary for the high definition may make it difficult to focus on the major subject to reduce the responsiveness.